Flexography is one method of printing words and images onto foil, plastic film, corrugated board, paper, paperboard, cellophane, or even fabric. In fact, since the flexographic process can be used to print on such a wide variety of materials, it is often the best graphic arts reproduction process for package printing.
The anilox cylinder serves as the heart of the flexographic press. The use of an ink-metering anilox cylinder, which is engraved with a cell pattern, enables an even and fast ink transfer to the printing plate. The configuration of the cells in the anilox roller, the pressure between the rollers, and the use of a doctor blade mechanism control the amount of ink transferred to the printing plate. The shape and volume of the cells are chosen to suit the anilox surface (chrome or ceramic), the doctoring system, the press capabilities, the printing substrate, and the image type (solid or halftone). Advances in anilox technology have resulted in laser-engraved ceramic anilox rollers offering tougher and longwearing rollers with greatly improved ink release characteristics compared to conventional mechanically engraved chrome roller technology.
Flexography prints can be made with a flexible printing plate that is wrapped around a rotating cylinder. The plate is usually made of natural or synthetic rubber or a photosensitive plastic material called photopolymer. It is usually attached to the plate cylinder with double-sided sticky tape. Flexography is a relief printing process, meaning that the image area on the printing plate is raised above the non-image area.
The image area receives the ink from the anilox roller, which is transferred to the print substrate when the latter is pressed with support of the impression cylinder against the printing plate. Flexography is a direct method, that is, the printing plate transfers the ink directly to the substrate.
Due to improved registration, the most popular type of press is the CI press (central-impression) where printing units are arranged around a single central impression cylinder.
In general, the higher the speed of the press, the wider the press will be. When the press is wider and faster, the diameter of the anilox roller must be greater in order to prevent damage to the roller due to deflection and bending. A 50-inch (ca 127 cm) machine has a 6-inch (ca 15 cm) diameter anilox cylinder. The dwell time between the chamber and the ink transfer nip is shorter.
Linear speeds in excess of 1800 ft/min (ca 0.549 km/min) are considered high speed for printing flexible substrates, and presses with the capability of printing at a linear speed of 3300 ft/min (ca 1 km/min) are now appearing on the market.
The linear speed of 3300 ft/min (ca 1 km/min) is equal to a linear velocity of 35 miles per hour (ca 56.3 km/hr), and conventional plates and the double-sided sticky tape will eject from the press at this speed. In place of plates and double-sided sticky tape, direct laser engraved elastomer sleeves are used for printing at these velocities. The usual chambered doctor blade has a two-inch gap between the blades, and the dwell time for this distance at 3300 ft/min (ca 1 km/min) is less than the time of a high speed shutter on a 35 mm camera. In that interval, the air must be displaced from the cells of the anilox, ink must enter the cells, and the air must be cycled out from the chamber.
At linear speeds up to 2300 ft/min (ca 0.701 km/min), normal motors can be used; however, at linear speeds over 2300 ft/min water-cooled motors are required.
Many printers require inks and coatings to print at high speeds in order to improve the cost effectiveness of their operations. Flexographic printing linear speeds generally range up to 2000 ft/min (ca 0.609 km/min), and that speed can be expected to increase. At increasing linear speeds, for example greater than 1200 ft/min (ca 0.366 km/min), and especially 1800 ft/per minute (ca 0.549 km/min), the printability of the ink begins to deteriorate and print defects can be observed. This defect can be described as uniformly dispersed, irregularly shaped missed areas of printing. These defects are believed to result from the inability of the ink to wet out the surfaces of the printing blanket or plate or the substrate, or from the distinct mechanistic demands associated with a high speed printing press configuration as discussed in the above paragraphs.
Gravure printing is an example of intaglio printing. It uses a depressed or sunken surface for the image so that the image areas is generally honey comb shaped cells or wells that are etched or engraved into a printing cylinder. The unetched areas of the cylinder represent the non-image or unprinted areas. The cylinder rotates through an ink bath and excess ink is wiped off the cylinder by a flexible steel doctor blade. The ink remaining in the recessed cells forms the image by direct transfer to the substrate (paper or other material) as it passes between the plate cylinder and the impression cylinder.
Gravure inks are fluid inks with a very low viscosity that allows them to be drawn into the engraved cells in the cylinder then transferred onto the substrate. Flexographic and gravure inks are very similar and the basic constituents are essentially the same.
The transfer of ink to the substrate is one of the most important factors affecting the quality of the final printed product. However, due to dynamics of linear high-speed presses, conventional inks used for slower speeds will breakdown at high speeds, creating print defects. Any print defect will negatively affect productivity and the inherent printing advantages of using linear high-speed presses.
Typical flexographic/gravure printing inks contain resins, solvents, colorants, and additives. The resins include rosin esters, polyamides, polyurethanes, nitrocellulose, and others. The solvents are often based on alcohols, acetates, glycol ethers, and possibly other solvent classes.
Suspensions form the backbone of several industrial materials such as coatings, inks, paints, ceramics and cosmetics. The control and improvement of the rheology and stability of such materials in chemical engineering processes have been a significant concern of scientists and engineers for decades. The solids loading or solids content of a suspension has also a major impact on its end-use properties, such as for instance, the quality of the coating color in later printing process or the color density in printing inks. As the solids content of a suspension is a critical factor in its rheological, process and end-use properties, it is of tremendous significance to obtain the optimum properties of the suspension providing the right solids content with the desired rheological, process and end-uses properties. The fact that there could be some impact of the solids content and its maximum threshold, the so-called maximum packing fraction, on rheology, process and end-use properties was recognize but has not been investigated, K. and van de Ven, T. G. M., “Comparisons of modified effective medium theory with experimental data on shear thinning of concentrated latex dispersions,” J. Rheol., 54, 1-26 (2010); Larson, R. G., The Structure and Rheology of Complex Fluids, (Oxford University Press Inc., New York, N.Y., 1999).
The traditional remedies to improve the print quality of a flexographic ink suggested lowering the viscosity of the existing low- and moderate-speed inks in order to obtain the corresponding version of the ink suitable for high-speed conditions. It was believed that reducing viscosity would lead to a better leveling and improve the pin holing. While it is true that pin holing is a result of poor leveling, reducing the ink viscosity by adding more solvent did not improve the pin holing issue.
In general, the solids content of a coating color or an ink system should provide the required end-use properties, such as the coverage and the color of the final product. To reach this requirement, coaters and printers face a dilemma: on one hand, the solids content should be beyond a minimum to provide the desired coverage and color; but on the other hand, runnability requirements limit the high end of solids content. Therefore, coaters and printers must compromise by trial and error in order to find a condition with high solids content system that has an acceptable runnability.
It has now been found that the Maximum Packing Fraction (MPF) is a parameter which allows one to optimize ink formulation. Here it is important to note that the important aspect in evaluating the runnability of a coating color or ink is not its solids content alone. Instead, the important aspect is how far the solids content is distant from the MPF. For example, two ink systems with the same solids content, e.g., 35 vol %, can have totally different runnability or end-use properties if they have different MPF's, e.g. 50 vol % and 65 vol %, respectively. The reason is that the one system with a MPF of 65 vol % will lose a significant amount of its solvent content when going from 35 vol % and getting close to a solid-like state of 65 vol %, viz. a 30 vol % difference, while the other system needs only to lose solvent in going from 30 vol % to 50 vol %, viz. 15 vol %. Thus, the second system needs to eliminate half the solvent amount that the first system must eliminate to get to the same solid-like condition. Such differences have major impacts on both runnability and the end-use properties. It has been found that the MPF can be used as a key parameter to design a formulation.
The impact of solids content on various runnability and end-use parameters are reported in the literature. Ascanio, G., Carreau, P. J., Reglat, O. and Tanguy, P. A., “Extensional properties of coating colors at high strain rates,” TAPPI Adv. Coat. Fundam. Symp. 8th, Chicago, Ill., 172-182 (May 8-10, 2003); Backfolk, K., Grankvist, T. and Triantafillopoulos, N., “Slip rheology of coating colors containing calcium carbonate pigments with narrow particle size distributions,” TAPPI Adv. Coat. Fundam. Symp., 8th, Chicago, Ill., 148-165 (May 8-10, 2003). However, the application of the MPF as a formulating tool to design a formulation leading to a superior runnability has not been recognized.